This invention relates to the field of catheters, especially catheters used for hemodialysis. In particular, the invention concerns a double-lumen catheter in which the lumens are disposed coaxially.
Double-lumen catheters for hemodialysis, and for other purposes, have been known for a long time. Some of these catheters have had coaxial lumens, as exemplified by U.S. Pat. Nos. 4,493,696, 4,099,528, 4,270,535, and 4,202,332. Other catheters of the prior art have had lumens with a D-shaped cross-section, with a planar septum separating the lumens. Examples of the latter structure are given in U.S. Pat. Nos. 4,134,402, 4,583,968, and 4,692,141.
For a given pressure, the rate of fluid flow, through a lumen of circular cross-section, is greater than the flow rate for a lumen of D-shaped cross-section of comparable area. Not only does the circular cross-section generate less turbulence, but the back pressure developed is also less, so that less force is required to push a given volume of fluid through the catheter. Moreover, a circular cross-section enjoys the advantage that it does not have internal crevices which can promote the formation of blood clots.
A lumen having an annular cross-section enjoys substantially the same advantages possessed by circular lumens. Like the circular cross-section, an annular cross-section also does not have sharp bends, corners, or crevices.
Because of the inherent advantages of lumens having circular cross-sections, many attempts have been made to develop a practical catheter having coaxial lumens. These attempts have not been entirely successful, for the reasons explained below.
One major problem with the coaxial catheters of the prior art is that they have had no acceptable means of connecting the inner tube to the outer tube at the distal end. Indeed, in many of the double-lumen coaxial catheters of the prior art, the distal end of the inner tube simply dangles within the outer tube, and is not anchored at all. Each time such a catheter is used, it is necessary to insert the inner tube into the outer tube. The inner tube must be withdrawn upon completion of a dialysis operation. This procedure of repeatedly inserting and withdrawing the inner tube is extremely cumbersome, since it is necessary, in effect, to dismantle the catheter after each dialysis operation. The procedure is also relatively expensive, because it is necessary to replace the inner tube each time one wants to use the catheter again. Thus, if one uses a single catheter to perform 20 dialysis operations, it is necessary to discard an inner tube 20 times, during the life of the catheter.
Another disadvantage of coaxial catheters of the prior art is their tendency to cause blockage of the outer lumen. Due to fluid flow into the catheter, an area of vacuum is likely to develop. If the inner tube is dangling freely within the outer tube, at its distal end, the inner tube may be pulled, due to the vacuum, towards the interior wall of the outer tube. The inner tube may thus block one or more of the holes providing access to the outer lumen, thereby reducing or even totally blocking blood flow into that lumen.
Yet another problem with coaxial catheters of the prior art is the risk of leakage. Since the inner tube is intended to be removed and inserted periodically, it is necessary to provide a hemostasis valve, and to pass the inner tube through that valve. The presence of such a valve creates an additional risk of leakage.
The above-described problems are alleviated by non-coaxial catheter designs, such as those having lumens of D-shaped cross-sections, as shown in the above-cited U.S. Pat. Nos. 4,134,402, 4,583,968, and 4,692,141. The catheters shown in the latter patents have lumens which do not dangle at their distal ends, and which are firmly supported within the catheter. However, these catheters have the disadvantages associated with lumens of non-circular cross-sections, as described above.
The present invention solves the problems inherent in the coaxial catheters of the prior art, and provides a practical double-lumen catheter having lumens of circular and annular cross-section. The catheter of the present invention has an inner lumen which is firmly and permanently mounted within the outer lumen, thereby providing a rigid and reliable structure. The catheter of the present invention therefore enjoys the advantages associated with circular lumens, while still being both safe and economical.